JSP programming topic 2 JSP core Two (El expression, El Operator and El Built-in objects in addition to four domain property spaces)

Tag: El expression gets data from a specified field the properties of the Ask Bean gets the array and the map and the elements in the list El operator in addition to the El built-in objects associated with four domain property spaces

1. El Expression:

-El,expression Language, Expression language, is an easy way to get data in a JSP page. The EL expression is a concept introduced from the JSP2.0 version.

-The basic syntax of the EL expression is simple: any static part of the JSP page can get the value of the specified expression in the form of ${expression}.

-It is important to note that El can only fetch data from four major domains. The order in which they are found is: The property value of the specified name is looked up from the four fields in the range from small to large. (Pagecontext,request,session,applicaiton)

Such as:

650) this.width=650; "src=" Https://s1.51cto.com/oss/201710/22/2bdadddc38c0f04b517fd896f06564dd.png "title=" 07.png "alt=" 2bdadddc38c0f04b517fd896f06564dd.png "/>

A. Get data from the specified domain:

-Finding the application domain space from the PageContext will reduce the efficiency of execution. If a property is determined to reside in a domain property space, you can specify to find it directly from that space. In this case, you need to take advantage of El's four built-in objects related to attribute space:

650) this.width=650; "src=" Https://s1.51cto.com/oss/201710/22/8a2d5715229bade5ca2265ab92fbf1e7.png "title=" 06.png "alt=" 8a2d5715229bade5ca2265ab92fbf1e7.png "/>

b, accessing the Bean's properties: (You can use the dot operator multiple times)

-El can get to the specified object property value by ${key. Property}. The actual call to the underlying is the Get method for the corresponding property of the object.

-Of course, it can also be obtained using ${key[' property '} or ${key["property"}, which is not commonly used.

650) this.width=650; "src=" Https://s2.51cto.com/oss/201710/22/08bb46877fa38f31faa5db871eb33ef5.png "title=" 08.png "alt=" 08bb46877fa38f31faa5db871eb33ef5.png "/>

c, get the elements in the array:

-El can get the element to the specified index by ${key [index]}. However, it is important to note that the system does not throw an array out-of-bounds exception if the element in the array does not exist for that specified index.

650) this.width=650; "src=" Https://s2.51cto.com/oss/201710/22/0601404f3d5191b92812009da673979f.png "title=" 09.png "alt=" 0601404f3d5191b92812009da673979f.png "/>

D. Get the elements in the list:

-the element of the specified index in the list can be obtained in the same way as the element in the get array, by ${key[Index]}. If no element of the specified index exists in the list, the system does not throw an out-of-bounds exception. (Cannot use El expression to access set because set is unordered, no index concept)

650) this.width=650; "src=" Https://s4.51cto.com/oss/201710/22/3b7f5bff44c6571a57c4bff2b500aded.png "title=" 10.png "alt=" 3b7f5bff44c6571a57c4bff2b500aded.png "/>

E, get the element in map: El through ${attributename.mapkey} can get the value of the specified key of the specified map:

650) this.width=650; "src=" Https://s5.51cto.com/oss/201710/22/8236202967279a1242fd4e55d4b6f44e.png "title=" 11.png "alt=" 8236202967279a1242fd4e55d4b6f44e.png "/>

2. El Operator:

-El expressions can perform various operations, where the commonly used operators are:

650) this.width=650; "src=" Https://s5.51cto.com/oss/201710/22/05dea0d3757abbf849700caf01257614.png "title=" 12.png "alt=" 05dea0d3757abbf849700caf01257614.png "/>

-In addition to the above operators, there is a very useful operator, empty, whose usage is the ${empty variable}, and the result is a Boolean value. If the variable is defined, the return value is true, if the variable is of type string and its value is a null string, the return value is true, if the variable is a reference type, and its value is NULL, the return value is "true" if the variable is of the collection type and it does not contain any elements.

650) this.width=650; "src=" Https://s4.51cto.com/oss/201710/22/2ce943a169e642f0408c829ecf1872da.png "title=" 13.png "alt=" 2ce943a169e642f0408c829ecf1872da.png "/>

3. In addition to the El built-in objects associated with four domain attribute spaces:

-Just like the Java code block of JSP and the expression block can use nine built-in objects, in the El expression, there is also a built-in object, and there are 11 built-in objects. Common built-in objects, in addition to the built-in objects related to the four domain property spaces previously used, are as follows:

A, PageContext:

-The PageContext is the same object as the PageContext in the JSP built-in object. Through this object, you can get to the request, response, session, ServletContext, ServletConfig and other objects. Note that these objects are not built-in objects in El. These objects can only be obtained through PageContext.

-Use ${pagecontext.request} directly in El to get the request object. Of course, the Pagecontext.getrequest () method is actually called at the bottom. Similarly, you can get to other objects in a similar way.

-Among these acquired objects, one is the most commonly used in the actual project: ${pagecontext.request.contextpath}, which is used to get the name of the current project published to the server, typically used in front of the path of the JSP page.

650) this.width=650; "src=" Https://s2.51cto.com/oss/201710/22/9800b27a6882f8e7c97f13d51adbf3df.png "title=" 14.png "alt=" 9800b27a6882f8e7c97f13d51adbf3df.png "/>

-In El's 11 built-in objects, the other 10 built-in objects, except PageContext, are of type Java.util.Map.

b, param:

The specified parameter value in the request is obtained by ${param. Parameter name} in El, and the underlying actual invocation is: Request.getparameter (), for example, the submitted request is:

650) this.width=650; "src=" Https://s2.51cto.com/oss/201710/22/6d14f278dfff8181be73511400aad7cb.png "style=" float: none; "title=" 15.png "alt=" 6d14f278dfff8181be73511400aad7cb.png "/>

-In the JSP page, you can get the value of the name parameter to ABC in the following way:

650) this.width=650; "src=" Https://s2.51cto.com/oss/201710/22/82ef45129690208dcdda16cf3baf48b7.png "style=" float: none; "title=" 16.png "alt=" 82ef45129690208dcdda16cf3baf48b7.png "/>

C, Paramvalues:

-If the requested request has more than one value in the same parameter, it can be obtained by ${paramvalues. Parameter name [index]} to the parameter value of the specified index number. For example, the submitted request is:

650) this.width=650; "src=" Https://s5.51cto.com/oss/201710/22/d64d9c5bcd29b22fc19426e952f2fc05.png "title=" 18.png "alt=" D64d9c5bcd29b22fc19426e952f2fc05.png "/>

-The specified parameter values in the request can be obtained in the JSP page as follows:

650) this.width=650; "src=" Https://s1.51cto.com/oss/201710/22/bc1425c079a8d10222674ba1d72f890a.png "title=" 19.png "alt=" Bc1425c079a8d10222674ba1d72f890a.png "/>

D. If you want to get the contents of sub-tags <param-value> in the <context-param> tag in Web. XML, you can pass the contents of the ${initparam.<param-name> tab} To obtain:

650) this.width=650; "src=" Https://s1.51cto.com/oss/201710/22/e5008b607837964ad73aedd4ef77832b.png "title=" 20.png "alt=" E5008b607837964ad73aedd4ef77832b.png "/>

-In the JSP page, get it in the following way:

650) this.width=650; "src=" Https://s5.51cto.com/oss/201710/22/f991595a936a6589359f5a28618a872a.png "title=" 21.png "alt=" F991595a936a6589359f5a28618a872a.png "/>

This article is from the "12392717" blog, please be sure to keep this source http://12402717.blog.51cto.com/12392717/1975027

JSP programming topic 2 JSP core Two (El expression, El Operator and El Built-in objects in addition to four domain property spaces)